ERK3 regulates TDP2-mediated DNA damage response and chemoresistance in lung cancer cells

Role of the Atypical MAPK ERK3 in Cancer Growth and Progression

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK) whose structural and regulatory features are distinct from those of conventional MAPKs, such as ERK1/2. Since its identification in 1991, the regulation, substrates and functions of ERK3 have remained largely unknown. However, recent years have witnessed a wealth of new findings about ERK3 signaling. Several important biological functions for ERK3 have been revealed, including its role in neuronal morphogenesis, inflammation, metabolism, endothelial cell tube formation and epithelial architecture. In addition, ERK3 has been recently shown to play important roles in cancer cell proliferation, migration, invasion and chemoresistance in multiple types of cancers. Furthermore, accumulating studies have uncovered various molecular mechanisms by which the expression level, protein stability and activity of ERK3 are regulated. In particular, several post-translational modifications (PTMs), including ubiquitination, hydroxylation and phosphorylation, have been shown to regulate the stability and activity of ERK3 protein. In this review, we discuss recent findings regarding biochemical and cellular functions of ERK3, with a main focus on its roles in cancers, as well as the molecular mechanisms of regulating its expression and activity.

Murine leukemia virus (MLV) P50 protein induces cell transformation via transcriptional regulatory function

BACKGROUND

The murine leukemia virus (MLV) has been a powerful model of pathogenesis for the discovery of genes involved in cancer. Its splice donor (SD')-associated retroelement (SDARE) is important for infectivity and tumorigenesis, but the mechanism remains poorly characterized. Here, we show for the first time that P50 protein, which is produced from SDARE, acts as an accessory protein that transregulates transcription and induces cell transformation.

RESULTS

By infecting cells with MLV particles containing SDARE transcript alone (lacking genomic RNA), we show that SDARE can spread to neighbouring cells as shown by the presence of P50 in infected cells. Furthermore, a role for P50 in cell transformation was demonstrated by CCK8, TUNEL and anchorage-independent growth assays. We identified the integrase domain of P50 as being responsible for transregulation of the MLV promoter using luciferase assay and RTqPCR with P50 deleted mutants. Transcriptomic analysis furthermore revealed that the expression of hundreds of cellular RNAs involved in cancerogenesis were deregulated in the presence of P50, suggesting that P50 induces carcinogenic processes via its transcriptional regulatory function.

CONCLUSION

We propose a novel SDARE-mediated mode of propagation of the P50 accessory protein in surrounding cells. Moreover, due to its transforming properties, P50 expression could lead to a cellular and tissue microenvironment that is conducive to cancer development.

Chemoresistant Cancer Cell Lines Are Characterized by Migratory, Amino Acid Metabolism, Protein Catabolism and IFN1 Signalling Perturbations

Simple Summary

While chemoresistance remains a major barrier to improving the outcomes for patients with ovarian cancer, the molecular features, and associated biological functions, which underpin chemoresistance in ovarian cancer remain poorly understood. In this study we aimed to provide insight into the proteins and metabolites, and their associated biological pathways, which play a role in conferring chemoresistance to ovarian cancer. Through mass spectrometry analysis comparing the proteome and metabolome of chemosensitive vs chemoresistant ovarian cancer cell lines we revealed numerous perturbations in signalling and metabolic pathways in chemoresistant cells. Further comparison to primary cells taken from patients with chemoresistant or chemosensitive disease identified a shared dysregulation in cytokine and type 1 interferon signalling. Our research sets the foundation for a deeper understanding of the proteomic and metabolomic features of chemoresistance and identifies type 1 interferon signalling as a common feature of chemoresistance.

Abstract

Chemoresistance remains the major barrier to effective ovarian cancer treatment. The molecular features and associated biological functions of this phenotype remain poorly understood. We developed carboplatin-resistant cell line models using OVCAR5 and CaOV3 cell lines with the aim of identifying chemoresistance-specific molecular features. Chemotaxis and CAM invasion assays revealed enhanced migratory and invasive potential in OVCAR5-resistant, compared to parental cell lines. Mass spectrometry analysis was used to analyse the metabolome and proteome of these cell lines, and was able to separate these populations based on their molecular features. It revealed signalling and metabolic perturbations in the chemoresistant cell lines. A comparison with the proteome of patient-derived primary ovarian cancer cells grown in culture showed a shared dysregulation of cytokine and type 1 interferon signalling, potentially revealing a common molecular feature of chemoresistance. A comprehensive analysis of a larger patient cohort, including advanced in vitro and in vivo models, promises to assist with better understanding the molecular mechanisms of chemoresistance and the associated enhancement of migration and invasion.

Bioinformatics Analyses Identify the Therapeutic Potential of ST8SIA6 for Colon Cancer

Sialylation of glycoproteins is modified by distinct sialyltransferases such as ST3Gal, ST6Gal, ST6GalNAc, or ST8SIA with α2,3-, α2,6-, or α2,8-linkages. Alteration of these sialyltransferases causing aberrant sialylation is associated with the progression of colon cancer. However, among the ST8- sialyltransferases, the role of ST8SIA6 in colon cancer remains poorly understood. In this study, we explored the involvement of ST8SIA6 in colon cancer using multiple gene databases. The relationship between ST8SIA6 expression and tumor stages/grades was investigated by UALCAN analysis, and Kaplan–Meier Plotter analysis was used to analyze the expression of ST8SIA6 on the survival outcome of colon cancer patients. Moreover, the biological functions of ST8SIA6 in colon cancer were explored using LinkedOmics and cancer cell metabolism gene DB. Finally, TIMER and TISMO analyses were used to delineate ST8SIA6 levels in tumor immunity and immunotherapy responses, respectively. ST8SIA6 downregulation was associated with an advanced stage and poorly differentiated grade; however, ST8SIA6 expression did not affect the survival outcomes in patients with colon cancer. Gene ontology analysis suggested that ST8SIA6 participates in cell surface adhesion, angiogenesis, and membrane vesicle trafficking. In addition, ST8SIA6 levels affected immunocyte infiltration and immunotherapy responses in colon cancer. Collectively, these results suggest that ST8SIA6 may serve as a novel therapeutic target towards personalized medicine for colon cancer.

MAPK6-AKT signaling promotes tumor growth and resistance to mTOR kinase blockade

Mitogen-activated protein kinase 6 (MAPK6) is an atypical MAPK. Its function in regulating cancer growth remains elusive. Here, we reported that MAPK6 directly activated AKT and induced oncogenic outcomes. MAPK6 interacted with AKT through its C34 region and the C-terminal tail and phosphorylated AKT at S473 independent of mTORC2, the major S473 kinase. mTOR kinase inhibitors have not made notable progress in the clinic. Our identified MAPK6-AKT axis may provide a major resistance pathway. Besides repressing growth, inhibiting MAPK6 sensitized cancer cells to mTOR kinase inhibitors. MAPK6 overexpression is associated with decreased overall survival and the survival of patients with lung adenocarcinoma, mesothelioma, uveal melanoma, and breast cancer. MAPK6 expression also correlated with AKT phosphorylation at S473 in human cancer tissues. We conclude that MAPK6 can promote cancer by activating AKT independent of mTORC2 and targeting MAPK6, either alone or in combination with mTOR blockade, may be effective in cancers.

Conditional ERK3 overexpression cooperates with PTEN deletion to promote lung adenocarcinoma formation in mice

ERK3, officially known as mitogen‐activated protein kinase 6 (MAPK6), is a poorly studied mitogen‐activated protein kinase (MAPK). Recent studies have revealed the upregulation of ERK3 expression in cancer and suggest an important role for ERK3 in promoting cancer cell growth and invasion in some cancers, in particular lung cancer. However, it is unknown whether ERK3 plays a role in spontaneous tumorigenesis in vivo. To determine the role of ERK3 in lung tumorigenesis, we created a conditional ERK3 transgenic mouse line in which ERK3 transgene expression is controlled by Cre recombinase. By crossing these transgenic mice with a mouse line harboring a lung tissue–specific Cre recombinase transgene driven by a club cell secretory protein gene promoter (CCSP‐iCre), we have found that conditional ERK3 overexpression cooperates with phosphatase and tensin homolog (PTEN) deletion to induce the formation of lung adenocarcinomas (LUADs). Mechanistically, ERK3 overexpression stimulates activating phosphorylations of erb‐b2 receptor tyrosine kinases 2 and 3 (ERBB2 and ERBB3) by upregulating Sp1 transcription factor (SP1)–mediated gene transcription of neuregulin 1 (NRG1), a potent ligand for ERBB2/ERBB3. Our study has revealed a bona fide tumor‐promoting role for ERK3 using genetically engineered mouse models. Together with previous findings showing the roles of ERK3 in cultured cells and in a xenograft lung tumor model, our findings corroborate that ERK3 acts as an oncoprotein in promoting LUAD development and progression.

Molecular docking and anticancer activity determination of 5,10-dihydro-7,8-dimethyl alloxazine derived from lumichrome of riboflavin

 Cancer is accountable for the demise of numerous lives worldwide annually. In this research a derivative of lumichrome, 5,10-dihydro-7,8-dimethyl alloxazine was assessed for its anticancer property through docking study. It was appraised after performing molecular docking study of the 5,10-dihydro-7,8-dimethyl alloxazine, there was a strong interaction between multiple oncogenic target proteins like CDK2/CCNE2 (–8.5 kcal/mol), TDP2 (–8 kcal/mol), NAD-SIRT2 (–10.9 kcal/mol) and lung cancer and acute lymphoblastic leukemia (ALL). Additionally, according to ADMET analysis, the synthesized compound 5,10-dihydro-7,8-dimethyl alloxazine also has good physicochemical characteristics to be a drug candidate. Consequently, these verdicts will assist the development of a novel anti-lung cancer and anti-leukemic agent which will eventually improve the endurance of cancer patients.

LncRNA GAS6-AS2 promotes non-small-cell lung cancer cell proliferation via regulating miR-144-3p/ MAPK6 axis

The function of a new long non-coding RNA GAS6-AS2 in non-small cell lung cancer (NSCLC) is not fully understood. In this study, GAS6-AS2 was identified, and its roles as well as mechanisms in regulating proliferation of NSCLCs cells were investigated. qRT-PCR was used to analyze GAS6-AS2, miR-144-3p, and MAPK6 expression. Protein expression was detected by Western blotting. Cell Counting Kit-8 (CCK8) assay was used to examine the cell proliferation ability. The interaction between GAS6-AS2 and miR-144-3p was confirmed by dual-luciferase reporter assay and RNA pull down assay. A xenograft model was constructed to monitor the mice NSCLC tumor growth in vivo. GAS6-AS2 was up-regulated, while miR-144-3p was suppressed in NSCLC cells compared with normal lung cells. GAS6-AS2 suppression could inhibit the progression of NSCLC cells, and miR-144-3p could attenuate the effect. GAS6-AS2 could function as a competitive endogenous RNA (ceRNA) via direct sponging miR-144-3p-3p, which further regulating the expression of MAPK6. The knockdown of GAS6-AS2 could greatly suppress the tumor growth of NSCLC in vivo. GAS6-AS2 up-regulated MAPK6 by sponging miR-144-3p in NSCLC tissues and cells. Thus, GAS6-AS2 is an effective therapeutic target in NSCLC.

Crystal Structure and Inhibitor Identifications Reveal Targeting Opportunity for the Atypical MAPK Kinase ERK3

Extracellular signal-regulated kinase 3 (ERK3), known also as mitogen-activated protein kinase 6 (MAPK6), is an atypical member of MAPK kinase family, which has been poorly studied. Little is known regarding its function in biological processes, yet this atypical kinase has been suggested to play important roles in the migration and invasiveness of certain cancers. The lack of tools, such as a selective inhibitor, hampers the study of ERK3 biology. Here, we report the crystal structure of the kinase domain of this atypical MAPK kinase, providing molecular insights into its distinct ATP binding pocket compared to the classical MAPK ERK2, explaining differences in their inhibitor binding properties. Medium-scale small molecule screening identified a number of inhibitors, several of which unexpectedly exhibited remarkably high inhibitory potencies. The crystal structure of CLK1 in complex with CAF052, one of the most potent inhibitors identified for ERK3, revealed typical type-I binding mode of the inhibitor, which by structural comparison could likely be maintained in ERK3. Together with the presented structural insights, these diverse chemical scaffolds displaying both reversible and irreversible modes of action, will serve as a starting point for the development of selective inhibitors for ERK3, which will be beneficial for elucidating the important functions of this understudied kinase.

Excision repair of topoisomerase DNA-protein crosslinks (TOP-DPC)

Topoisomerases are essential enzymes solving DNA topological problems such as supercoils, knots and catenanes that arise from replication, transcription, chromatin remodeling and other nucleic acid metabolic processes. They are also the targets of widely used anticancer drugs (e.g. topotecan, irinotecan, enhertu, etoposide, doxorubicin, mitoxantrone) and fluoroquinolone antibiotics (e.g. ciprofloxacin and levofloxacin). Topoisomerases manipulate DNA topology by cleaving one DNA strand (TOP1 and TOP3 enzymes) or both in concert (TOP2 enzymes) through the formation of transient enzyme-DNA cleavage complexes (TOPcc) with phosphotyrosyl linkages between DNA ends and the catalytic tyrosyl residue of the enzymes. Failure in the self-resealing of TOPcc results in persistent TOPcc (which we refer it to as topoisomerase DNA-protein crosslinks (TOP-DPC)) that threaten genome integrity and lead to cancers and neurodegenerative diseases. The cell prevents the accumulation of topoisomerase-mediated DNA damage by excising TOP-DPC and ligating the associated breaks using multiple pathways conserved in eukaryotes. Tyrosyl-DNA phosphodiesterases (TDP1 and TDP2) cleave the tyrosyl-DNA bonds whereas structure-specific endonucleases such as Mre11 and XPF (Rad1) incise the DNA phosphodiester backbone to remove the TOP-DPC along with the adjacent DNA segment. The proteasome and metalloproteases of the WSS1/Spartan family typify proteolytic repair pathways that debulk TOP-DPC to make the peptide-DNA bonds accessible to the TDPs and endonucleases. The purpose of this review is to summarize our current understanding of how the cell excises TOP-DPC and why, when and where the cell recruits one specific mechanism for repairing topoisomerase-mediated DNA damage, acquiring resistance to therapeutic topoisomerase inhibitors and avoiding genomic instability, cancers and neurodegenerative diseases.

Repair of DNA-protein crosslinks in plants

DNA-protein crosslinks represent a severe kind of DNA damage as they disturb essential processes, such as transcription and DNA replication, due to their bulkiness. To ensure the maintenance of genome integrity, it is necessary for all living organisms to repair these lesions in a timely manner. Over recent years, much knowledge has been obtained regarding the repair of DNA-protein crosslinks (DPC), but it was only recently that the first insights into the mechanisms of DPC repair in plants were obtained. The plant DPC repair network consists of at least three parallel pathways that resolve DPC by distinct biochemical mechanisms. The endonuclease MUS81 resolves the DPC by cleaving the DNA part of the crosslink, the protease WSS1A is able to degrade the protein part and the tyrosyl-DNA-phosphodiesterase TDP1 can hydrolyse the crosslink between a protein and the DNA. However, due to the variety of different DPC types and the evolutionary conservation of pathways between eukaryotes, we expect that future research will reveal additional factors involved in DPC repair in plants.

Molecular mechanisms of topoisomerase 2 DNA-protein crosslink resolution

The compaction of DNA and the continuous action of DNA transactions, including transcription and DNA replication, create complex DNA topologies that require Type IIA Topoisomerases, which resolve DNA topological strain and control genome dynamics. The human TOP2 enzymes catalyze their reactions via formation of a reversible covalent enzyme DNA-protein crosslink, the TOP2 cleavage complex (TOP2cc). Spurious interactions of TOP2 with DNA damage, environmental toxicants and chemotherapeutic "poisons" perturbs the TOP2 reaction cycle, leading to an accumulation of DNA-protein crosslinks, and ultimately, genomic instability and cell death. Emerging evidence shows that TOP2-DNA protein crosslink (DPC) repair entails multiple strand break repair activities, such as removal of the poisoned TOP2 protein and rejoining of the DNA ends through homologous recombination (HR) or non-homologous end joining (NHEJ). Herein, we discuss the molecular mechanisms of TOP2-DPC resolution, with specific emphasis on the recently uncovered ZATTZnf451-licensed TDP2-catalyzed TOP2-DPC reversal mechanism.

Tyrosyl-DNA phosphodiesterases: rescuing the genome from the risks of relaxation

Tyrosyl–DNA Phosphodiesterases 1 (TDP1) and 2 (TDP2) are eukaryotic enzymes that clean-up after aberrant topoisomerase activity. While TDP1 hydrolyzes phosphotyrosyl peptides emanating from trapped topoisomerase I (Top I) from the 3′ DNA ends, topoisomerase 2 (Top II)-induced 5′-phosphotyrosyl residues are processed by TDP2. Even though the canonical functions of TDP1 and TDP2 are complementary, they exhibit little structural or sequence similarity. Homozygous mutations in genes encoding these enzymes lead to the development of severe neurodegenerative conditions due to the accumulation of transcription-dependent topoisomerase cleavage complexes underscoring the biological significance of these enzymes in the repair of topoisomerase–DNA lesions in the nervous system. TDP1 can promiscuously process several blocked 3′ ends generated by DNA damaging agents and nucleoside analogs in addition to hydrolyzing 3′-phosphotyrosyl residues. In addition, deficiency of these enzymes causes hypersensitivity to anti-tumor topoisomerase poisons. Thus, TDP1 and TDP2 are promising therapeutic targets and their inhibitors are expected to significantly synergize the effects of current anti-tumor therapies including topoisomerase poisons and other DNA damaging agents. This review covers the structural aspects, biology and regulation of these enzymes, along with ongoing developments in the process of discovering safe and effective TDP inhibitors.

Mammalian Tyrosyl-DNA Phosphodiesterases in the Context of Mitochondrial DNA Repair

Mammalian mitochondria contain four topoisomerases encoded in the nuclear genome: TOP1MT, TOP2α, TOP2β, and TOP3α. They also contain the two known tyrosyl-DNA phosphodiesterases (TDPs): TDP1 and TDP2, including a specific TDP2^S isoform. Both TDP1 and TDP2 excise abortive topoisomerase cleavage complexes (TOPccs), yet their molecular structures and mechanisms are different. TDP1 is present across eukaryotes, from yeasts to humans and belongs to the phospholipase D family. It functions without a metal cofactor and has a broad activity range, as it also serves to cleanse blocking 3′-DNA ends bearing phosphoglycolate, deoxyribose phosphate, nucleoside, nucleoside analogs (zidovudine), abasic moieties, and with a lower efficiency, TOP2ccs. Found in higher vertebrates, TDP2 is absent in yeast where TDP1 appears to perform its functions. TDP2 belongs to the exonuclease/endonuclease/phosphodiesterase family and requires magnesium as a cofactor to excise TOP2ccs, and it also excises TOP1ccs, albeit with a lower efficiency. Here, we review TDP1 and TDP2 in the context of mitochondrial DNA repair and discuss potential new research areas centered on the mitochondrial TDPs.

Germ Line Deletion Reveals a Nonessential Role of Atypical Mitogen-Activated Protein Kinase 6/Extracellular Signal-Regulated Kinase 3

Mitogen-activated protein kinase 6/extracellular signal-regulated kinase 3 (MAPK6/ERK3) is an atypical member of the MAPKs. An essential role has been suggested by the perinatal lethal phenotype of ERK3 knockout mice carrying a lacZ insertion in exon 2 due to pulmonary dysfunction and by defects in function, activation, and positive selection of T cells. To study the role of ERK3 in vivo, we generated mice carrying a conditional Erk3 allele with exon 3 flanked by loxP sites. Loss of ERK3 protein was validated after deletion of Erk3 in the female germ line using zona pellucida 3 (Zp3)-cre and a clear reduction of the protein kinase MK5 is detected, providing the first evidence for the existence of the ERK3/MK5 signaling complex in vivo In contrast to the previously reported Erk3 knockout phenotype, these mice are viable and fertile and do not display pulmonary hypoplasia, acute respiratory failure, abnormal T-cell development, reduction of thymocyte numbers, or altered T-cell selection. Hence, ERK3 is dispensable for pulmonary and T-cell functions. The perinatal lethality and lung and T-cell defects of the previous ERK3 knockout mice are likely due to ERK3-unrelated effects of the inserted lacZ-neomycin resistance cassette. The knockout mouse of the closely related atypical MAPK ERK4/MAPK4 is also normal, suggesting redundant functions of both protein kinases.

A Radioactive in vitro ERK3 Kinase Assay

Mitogen-activated protein kinases (MAPKs) are serine/threonine kinases that have an important role in signal transduction. Extracellular signal-regulated kinase 3 (ERK3), also known as MAPK6, is an atypical MAPK. Here, we describe in detail an in vitro assay for the kinase activity of ERK3 using myelin basic protein (MBP) or steroid receptor coactivator-3 (SRC-3) as substrates. The assay is carried out in the presence of [y-³²P]-ATP which results in radiolabeling of phosphorylated substrates. Separation of the reaction components by gel electrophoresis followed by autoradiography enables detection of the radiolabeled products, and hence determination of the kinase activity of ERK3. This assay can be used for several applications including identification of substrates, determination of the effect of molecules or mutations on kinase activity, and testing specific kinase inhibitors. Furthermore, the protocol outlined here can be adapted to measure the activity of other kinases by using their specific substrates.

An internal ribosome entry site in the coding region of tyrosyl-DNA phosphodiesterase 2 drives alternative translation start

Tyrosyl-DNA phosphodiesterase 2 (TDP2) is a multifunctional protein that has been implicated in a myriad of cellular pathways. Although most well-known for its phosphodiesterase activity removing stalled topoisomerase 2 from DNA, TDP2 has also been shown to interact with both survival and apoptotic mitogen-activated protein kinase (MAPK) signaling cascades. Moreover, it facilitates enterovirus replication and has been genetically linked to neurological disorders ranging from Parkinson's disease to dyslexia. To accurately evaluate TDP2 as a therapeutic target, we need to understand how TDP2 performs such a wide diversity of functions. Here, we use cancer cell lines modified with CRISPR/Cas9 or stably-expressed TDP2-targeted shRNA and transfection of various TDP2 mutants to show that its expression is regulated at the translational level via an internal ribosome entry site (IRES) that initiates translation at codon 54, the second in-frame methionine of the TDP2 coding sequence. We observed that this IRES drives expression of a shorter, N-terminally truncated isoform of TDP2, ΔN-TDP2, which omits a nuclear localization sequence. Additionally, we noted that ΔN-TDP2 retains phosphodiesterase activity and is protective against etoposide-induced cell death, but co-immunoprecipitates with fewer high-molecular-weight ubiquitinated peptide species, suggesting partial loss-of-function of TDP2's ubiquitin-association domain. In summary, our findings suggest the existence of an IRES in the 5' coding sequence of TDP2 that translationally regulates expression of an N-terminally truncated, cytoplasmic isoform of TDP2. These results shed light on the regulation of this multifunctional protein and may inform the design of therapies targeting TDP2 and associated pathways.

The atypical MAPK ERK3 potently suppresses melanoma cell growth and invasiveness

Mitogen-activated protein kinase 6 (MAPK6) represents an atypical MAPK also known as extracellular signal-regulated kinase 3 (ERK3), which has been shown to play roles in cell motility and metastasis. ERK3 promotes migration and invasion of lung cancer cells and head and neck cancer cells by regulating the expression and/or activity of proteins involved in cancer progression. For instance, ERK3 upregulates matrix metallopeptidases and thereby promotes cancer cell invasiveness, and it phosphorylates tyrosyl-DNA phosphodiesterase 2, thereby enhancing chemoresistance in lung cancer. Here we discovered that ERK3 plays a converse role in melanoma. We observed that BRAF, an oncogenic Ser/Thr kinase, upregulates ERK3 expression levels by increasing both ERK3 messenger RNA levels and protein stability. Interestingly, although BRAF's kinase activity was required for upregulating ERK3 gene transcription, BRAF stabilized ERK3 protein in a kinase-independent fashion. We further demonstrate that ERK3 inhibits the migration, proliferation and colony formation of melanoma cells. In line with this, high level of ERK3 predicted increased survival among patients with melanomas. Taken together, these results indicate that ERK3 acts as a potent suppressor of melanoma cell growth and invasiveness.

Activation loop phosphorylation of ERK3 is important for its kinase activity and ability to promote lung cancer cell invasiveness

ERK3 is an atypical mitogen-activated protein kinase (MAPK) that has recently gained interest for its role in promoting cancer cell migration and invasion. However, the molecular regulation of ERK3 functions in cancer cells is largely unknown. ERK3 has a single phospho-acceptor site (Ser¹⁸⁹) in its activation motif rather than the TXY conserved in conventional MAPKs such as ERK1/2. Although dual phosphorylation of the TXY motif is known to be critical for the activation of conventional MAPKs, the role of Ser¹⁸⁹ phosphorylation in ERK3 activity and its function in cancer cells remain elusive. In this study, we revealed that activation loop phosphorylation is important for ERK3 in promoting cancer cell invasiveness, as the S189A mutation greatly decreased the ability of ERK3 to promote migration and invasion of lung cancer cells. Interestingly, a catalytically inactive ERK3 mutant was still capable of increasing migration and invasion, although to a lesser extent compared with WT ERK3, suggesting that ERK3 promotes cancer cell invasiveness by both kinase-dependent and kinase-independent mechanisms. To elucidate how the S189A mutation reduces the invasiveness-promoting ability of ERK3, we tested its effect on the kinase activity of ERK3 toward steroid receptor coactivator 3 (SRC3), a recently identified substrate of ERK3 critical for cancer cell invasiveness. Compared with ERK3, ERK3-S189A exhibited a dramatic decrease in kinase activity toward SRC3 and a concomitantly reduced ability to stimulate matrix metalloproteinase expression. Taken together, our study unravels the importance of Ser¹⁸⁹ phosphorylation for intramolecular regulation of ERK3 kinase activity and invasiveness-promoting ability in lung cancer cells.

The atypical mitogen-activated protein kinase ERK3 is essential for establishment of epithelial architecture

Epithelia contribute to physical barriers that protect internal tissues from the external environment and also support organ structure. Accordingly, establishment and maintenance of epithelial architecture are essential for both embryonic development and adult physiology. Here, using gene knockout and knockdown techniques along with gene profiling, we show that extracellular signal-regulated kinase 3 (ERK3), a poorly characterized atypical mitogen-activated protein kinase (MAPK), regulates the epithelial architecture in vertebrates. We found that in Xenopus embryonic epidermal epithelia, ERK3 knockdown impairs adherens and tight-junction protein distribution, as well as tight-junction barrier function, resulting in epidermal breakdown. Moreover, in human epithelial breast cancer cells, inhibition of ERK3 expression induced thickened epithelia with aberrant adherens and tight junctions. Results from microarray analyses suggested that transcription factor AP-2α (TFAP2A), a transcriptional regulator important for epithelial gene expression, is involved in ERK3-dependent changes in gene expression. Of note, TFAP2A knockdown phenocopied ERK3 knockdown in both Xenopus embryos and human cells, and ERK3 was required for full activation of TFAP2A-dependent transcription. Our findings reveal that ERK3 regulates epithelial architecture, possibly together with TFAP2A.

L290P/V mutations increase ERK3's cytoplasmic localization and migration/invasion-promoting capability in cancer cells

Protein kinases are frequently mutated in human cancers, which leads to altered signaling pathways and contributes to tumor growth and progression. ERK3 is an atypical mitogen-activated protein kinase (MAPK) containing an S-E-G activation motif rather than the conserved T-X-Y motif in conventional MAPKs such as ERK1/2. Recent studies have revealed important roles for ERK3 in cancers. ERK3 promotes cancer cell migration/invasion and tumor metastasis, and its expression is upregulated in multiple cancers. Little is known, however, regarding ERK3 mutations in cancers. In the present study, we functionally and mechanistically characterized ERK3 L290P/V mutations, which are located within ERK3’s kinase domain, and are shown to exist in several cancers including lung cancer and colon cancer. We found that in comparison with wild type ERK3, both L290P and L290V mutants have greatly increased activity in promoting cancer cell migration and invasion, but have little impact on ERK3’s role in cell proliferation. Mechanistically, while they have no clear effect on kinase activity, L290P/V mutations enhance ERK3’s cytoplasmic localization by increasing the interaction with the nuclear export factor CRM1. Our findings suggest that L290P/V mutations of ERK3 may confer increased invasiveness to cancers.

Regulatory players of DNA damage repair mechanisms: Role in Cancer Chemoresistance

DNA damaging agents are most common in chemotherapeutic molecules that act against cancer. However, cancer cells possess inherent biological features to overcome DNA damages by activating various distinct repair mechanisms and pathways. Importantly, various oncogenes, cancer stem cells (CSCs), hypoxic environment, transcription factors and bystander signaling that are activated in the cancer cells influence DNA repair, thereby effectively repairing the DNA damage. Repaired cancer cells often become more resistance to further therapy and results in disease recurrence. In this review, we summarize how the various signaling pathways in cancer cells regulates DNA repair and induce chemoresistance.

Deubiquitinating Enzyme USP20 Regulates Extracellular Signal-Regulated Kinase 3 Stability and Biological Activity

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK) whose regulatory mechanisms and biological functions remain superficially understood. Contrary to most protein kinases, ERK3 is a highly unstable protein that is subject to dynamic regulation by the ubiquitin-proteasome system. However, the effectors that control ERK3 ubiquitination and degradation are unknown. In this study, we carried out an unbiased functional loss-of-function screen of the human deubiquitinating enzyme (DUB) family and identified ubiquitin-specific protease 20 (USP20) as a novel ERK3 regulator. USP20 interacts with and deubiquitinates ERK3 both in vitro and in intact cells. The overexpression of USP20 results in the stabilization and accumulation of the ERK3 protein, whereas USP20 depletion reduces the levels of ERK3. We found that the expression levels of ERK3 correlate with those of USP20 in various cellular contexts. Importantly, we show that USP20 regulates actin cytoskeleton organization and cell migration in a manner dependent on ERK3 expression. Our results identify USP20 as a bona fide regulator of ERK3 stability and physiological functions.

A regulatory BMI1/let-7i/ERK3 pathway controls the motility of head and neck cancer cells

Extracellular signal‐regulated kinase 3 (ERK3) is an atypical mitogen‐activated protein kinase (MAPK), whose biological activity is tightly regulated by its cellular abundance. Recent studies have revealed that ERK3 is upregulated in multiple cancers and promotes cancer cell migration/invasion and drug resistance. Little is known, however, about how ERK3 expression level is upregulated in cancers. Here, we have identified the oncogenic polycomb group protein BMI1 as a positive regulator of ERK3 level in head and neck cancer cells. Mechanistically, BMI1 upregulates ERK3 expression by suppressing the tumor suppressive microRNA (miRNA) let‐7i, which directly targets ERK3 mRNA. ERK3 then acts as an important downstream mediator of BMI1 in promoting cancer cell migration. Importantly, ERK3 protein level is positively correlated with BMI1 level in head and neck tumor specimens of human patients. Taken together, our study revealed a molecular pathway consisting of BMI1, miRNA let‐7i, and ERK3, which controls the migration of head and neck cancer cells, and suggests that ERK3 kinase is a potential new therapeutic target in head and neck cancers, particularly those with BMI1 overexpression.

Role of Protein Linked DNA Breaks in Cancer

Topoisomerases are a group of specialized enzymes that function to maintain DNA topology by introducing transient DNA breaks during transcription and replication. As a result of abortive topoisomerases activity, topoisomerases catalytic intermediates may be trapped on the DNA forming topoisomerase cleavage complexes (Topcc). Topoisomerases trapping on the DNA is the mode of action of several anticancer drugs, it lead to formation of protein linked DAN breaks (PDBs). PDBs are now considered as one of the most dangerous forms of endogenous DNA damage and a major threat to genomic stability. The repair of PDBs involves both the sensing and repair pathways. Unsuccessful repair of PDBs leads to different signs of genomic instabilities such as chromosomal rearrangements and cancer predisposition. In this chapter we will summarize the role of topoisomerases induced PDBs, identification and signaling, repair, role in transcription. We will also discuss the role of PDBs in cancer with a special focus on prostate cancer.

Tyrosyl-DNA phosphodiesterase inhibitors: Progress and potential

DNA topoisomerases are essential during transcription and replication. The therapeutic mechanism of action of topoisomerase inhibitors is enzyme poisoning rather than catalytic inhibition. Tyrosyl-DNA phosphodiesterases 1 or 2 were found as DNA repair enzymes hydrolyzing the covalent bond between the tyrosyl residue of topoisomerases I or II and the 3'- or 5'-phosphate groups in DNA, respectively. Tyrosyl-DNA phosphodiesterase 1 is a key enzyme in DNA repair machinery and a promising target for antitumor and neurodegenerative therapy. Inhibitors of tyrosyl-DNA phosphodiesterase 1 could act synergistically with topoisomerase I inhibitors and thereby potentiate the effects of topoisomerase I poisons. Tyrosyl-DNA phosphodiesterase 2 is an enzyme that specifically repairs DNA damages induced by topoisomerase II poisons and causes resistance to these drugs. Selective inhibition of tyrosyl-DNA phosphodiesterase 2 may be a novel approach to overcome intrinsic or acquired resistance to topoisomerase II-targeted drug therapy. Thus, agents that inhibit tyrosyl-DNA phosphodiesterases 1 and 2 have many applications in biochemical and physiological research and they have the potential to become anticancer and antiviral drugs. The structures, mechanism of action and therapeutic rationale of tyrosyl-DNA phosphodiesterase inhibitors and their development for combinations with topoisomerase inhibitors and DNA damaging agents are discussed.